Prevent pushing latest Docker image unless actually latest

[code-asher]

Add a check before pushing the Docker image from CI to see whether it is later than the latest version on Dockerhub. If it is, then we also push the latest tag.

This may not happen in practice (requires releasing an old version that has not already been uploaded to Dockerhub) but seems like something we could make bulletproof.

See the chat here: https://github.com/cdr/code-server/pull/4120/files#r704712372

[code-asher]

Also maybe we should always push the version tag to Dockerhub even if already pushed because if we are rebuilding/re-releasing an old version that is probably what we want?

[code-asher]

Then again we might be rebuilding because some other part of the workflow failed (npm, etc) and we do not need to rebuild the image and push to Docker (that was the original reason for the check). So maybe only make the latest change.

[stale]

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